This invention relates to a nickel-base superalloy article having a protective layer containing aluminum and a reactive element deposited on its surface, with the carbon content reduced by decarburizing.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan blades. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F.
Many approaches have been used to protect the turbine blades and vanes against the highly aggressive combustion-gas environment and to increase the operating temperature limit of the turbine blades and vanes. For example, the composition and processing of the base materials themselves have been improved. Physical cooling techniques may also be used.
In another approach, the surfaces of the turbine blades and vanes are coated with aluminum-containing protective coatings that protect the articles against the combustion gas, and in some cases insulate the articles from the temperature of the combustion gas. The articles are thereby able to run cooler and be more resistant to environmental attack.
The addition of selected elements to the protective coatings may improve the mechanical and environmental properties of the coatings. However, those results have not always been consistent, and there is a large scatter in the data. Even though there has been an indication of improved performance as a result of the presence of such elements, those improvements cannot be relied upon in all cases.
There is a need for an approach to improving the properties obtained in nickel-base superalloys having a protective coating. The present invention fulfills this need, and further provides related advantages.
The present invention provides a procedure that improves the performance of a nickel-base superalloy having a protective coating applied to its surface, and an article having this improved performance. The protective coating contains aluminum and a reactive element such as hafnium, zirconium, yttrium, lanthanum, and/or cerium. The procedure is readily performed with available apparatus, and may be integrated into the coating process. The coating protects the surface of the article against environmental damage, as in the case of conventional protective coatings.
A method for preparing a surface-protected article includes providing an article substrate having a surface and having a nominal bulk composition comprising a nickel-base superalloy. The nickel-base superalloy has more nickel than any other element, and a nominal bulk composition of carbon. The method further includes depositing a protective layer overlying the surface of the article substrate, wherein the protective layer comprises aluminum and a reactive element selected from the group consisting of hafnium, zirconium, yttrium, lanthanum, and cerium, and combinations thereof. The step of depositing a protective layer includes the steps of decarburizing locations where the carbon may serve as a barrier to the mobility of the reactive elements within the protective layer by scavenging the reactive elements and preventing their diffusion in the protective layer, and depositing the protective layer overlying the substrate. The protective layer may be an overlay coating or a diffusion coating. A ceramic layer may be deposited over the protective layer.
The reactive elements (hafnium, zirconium, yttrium, lanthanum, and cerium, and combinations thereof) present in the protective layer yield their greatest benefits when they are in solid solution and free to diffuse within the coating. The impurity element carbon chemically combines with the reactive elements to form compounds that remove the reactive elements from solid solution and thence prevent them from moving within the protective layer. The result is that their advantageous effects are reduced or lost. In the present approach, the carbon which may chemically combine with the reactive elements of the protective layer is partially removed so as to lessen its concentration. The carbon is preferably removed not only from the protective layer itself, but also from the surface region of the substrate, because it may diffuse from the substrate into the protective layer during service.
In practicing the method, the reducing of the carbon content is preferably accomplished by contacting a decarburizing agent to the surface of the substrate to decarburize to a depth of from about 5 to about 100 micrometers, decarburizing a platinum-containing layer after deposition (where the protective layer is a platinum aluminide), depositing the aluminum-containing layer from an atmosphere containing a reducing agent, and/or decarburizing the substrate and protective layer after it is deposited. The decarburizing agent is preferably a reducing gas such as hydrogen or carbon dioxide. Particularly in the case of the overlay protective layer, the starting materials of the protective layer may be decarburized prior to deposition.
The present approach provides a low-carbon region in the protective layer and in the substrate adjacent to the surface where the protective layer is deposited. The low carbon content of the protective layer results in the reactive elements not chemically combining with carbon to produce carbides of the reactive elements, so that the reactive elements remain free to diffuse throughout the protective layer. Such carbides reduce the level of the solute reactive element that is available to strengthen and improve the environmental properties of the coating. However, it is desirable to remove carbon from the surface region of the substrate as well, so that this surface region cannot serve as a diffusion source of carbon into the protective layer during service. The result is improved performance of the coating during service.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.